1. Technical Field
The present invention relates to a half mirror and an image display apparatus including the half mirror.
2. Related Art
In typical, image display apparatuses, such as head mount displays, head up displays, and viewfinders of camcorders, include half mirrors to split a luminous flux into a plurality of beams. In the optical system of an observation system included in an image-recording apparatus, a half mirror reflects light of an image displayed on a display device, such as a liquid crystal display device, to eyes of a viewer while transmitting the external light, so that the viewer can view both image information displayed on the display device and external light in one visual field.
Examples of such a half mirror include a prism-type beam splitter including transparent substrates bonded to each other. In the prism-type beam splitter, a thin film composed of metal such as Ag is formed on a surface of one transparent substrate, and the other transparent substrate having a refractive index the same as that of one transparent substrate is bonded to the topmost layer of the thin layer such that the thin film is interposed between the two transparent substrates, which enables transmitted light to directly travel and enables external distortion of a transmission image to be reduced.
Example of the thin film include a multilayered dielectric film including a high-refractive-index film and low-refractive-index film composed of dielectric materials and a structure including dielectric films and a metallic film interposed therebetween (see, U.S. Pat. No. 3,559,090). The latter, in which a metallic film is interposed between dielectric films, can reduce polarization dependence and incident angle dependence within a broad wavelength range being approximately entire visible light region and is therefore suitable for use in see-through optical systems which enable image information to be displayed while allowing observation of transmitted external light.
Another metal, such as Al, may be used in place of Ag to form the thin film used in a half mirror; however, a problem of large optical loss by absorption is caused, and it is difficult to precisely and uniformly form a significantly thin layer having a thickness of 1 to 5 nm. The thin Ag film also has a problem in which Ag is readily allowed to react for degradation, such as diffusion by heat, granulation, and oxidation with a gas composition.
In typical, in order to suppress such reactivity of Ag, the film needs to be formed on a substrate kept at low temperature under high vacuum, and a protective layer needs to be formed to suppress degradation in air. In other known techniques, Ag is alloyed with a small amount of a stabilizer without impairing the optical properties of Ag, or an adjacent metallic layer composed of, for example, Cr is formed so as to have a slightly thin thickness for the purpose of stable film formation as is disclosed in Japanese Patent No. 3563955.
In display apparatuses, however, a resin substrate is used to form a prism in some cases because of demands for reductions in the weight of an optical component and its production cost. The resin substrate has particular water absorbability and contains unstable molecules, and moisture and organic components are therefore leaked from the resin substrate and then volatilized to be taken into the film during film formation under vacuum. Thus, density of the Ag layer and the dielectric films with the Ag layer interposed therebetween is reduced, which problematically causes diffusion or degradation in the Ag layer with ease as compared with use of the traditional glass substrates.